The mitigation of induced electrical potentials on underground pipes and other electrical conductors has been addressed by others in the past. The induced electrical potentials on these underground pipes and conductors can be caused by a myriad of sources, including electrical currents and electrical potentials caused by the transmission of power through the underground pipe itself, electrical potentials from nearby transmission cables or overhead transmission lines, radio transmission antenna or towers, and other similar sources. The electrical potentials induced on underground pipes and other underground conductors may be extremely dangerous to persons coming in close proximity to the underground conductor. A person coming within close proximity may provide an electrical path to an area of lesser electrical potential causing a discharge of the electrical potential in the buried conductor such that the person is electrocuted.
Utility workers, in particular, regularly face the danger of electrocution from electrical potentials and induced alternating currents from transmission cables and induced potentials on underground conductors such as gas pipes. Most buried pipes and buried transmission cables have equipment connected to the buried conductor which is used to monitor, test, or perform maintenance and repairs to the underground conductor. For example, some underground gas pipes have metering stations throughout the length of the pipe to monitor the flow of gas. Some electrical transmission cables have metering stations to monitor oil pressure and oil temperature in the transmission cable and to detect fault conditions. These stations are typically above grade, at a ground-level surface above the buried conductor and may include access to the buried conductor or may include equipment which is connected to the buried conductor. Often times, the connections to the buried conductor are themselves conductors. For example, some underground utilities require test stands, valve sites, metering stations, pig launchers and receivers, access portals, or other exposed, above-ground equipment which are electrically connected to the underground conductor.
In order to protect persons coming in proximity to or contact with the underground conductor or with any such exposed, above-ground equipment, it is necessary to mitigate the magnitude of the electrical potentials at these sites. Sometimes it is also important to mitigate these electrical potentials to avoid damage to sensitive equipment used in close proximity to an underground conductor.
Prior technology to reduce electrical potentials at such access sites have included the use of grounding rods and interconnecting conductors typically custom made by workers at the desired location. This type of protective scheme led to the use of gradient control wires or conductors. Gradient control wires are set up in a matrix or array near the area needing mitigation of electric potentials. The gradient control wires act on electric potentials in the soil and earth surrounding the buried gradient control wires to bring the electric potential in the area around the wires closer to the potential of the underground conductor. This decreases the electric potential between the underground conductor and the surrounding soil near the buried wires. Hence, the voltage measured between the buried conductor and the work area are brought within acceptable, safe levels.
The use of a matrix of gradient control wires has been used and is known in the art. For example, U.S. Pat. No. 4,114,977 to Polidori discloses a connector for joining grounding grid wires at their nodal points of intersection. The grounding grids consist of a matrix or crossover network of conductors buried underground and connected to above ground equipment and buried grounding rods. Such grounding grids also serve to quickly dissipate fault current as well as induced currents.
Another example of a custom made gradient control grid is one sold by Platt Brothers & Co., Inc. This company produces a PLATTLINE zinc ribbon used to dissipate induced currents on underground pipes. The PLATTLINE zinc ribbon product may be installed in a grid-like configuration by laying out cut lengths of the ribbon in a grid pattern and then the points of intersection are either crimped together with copper rings or welded in place.
However, the use of gradient control wires and grids for step voltage and touch voltage mitigation has always involved the custom installation of the wires by workers in the field. It has involved cutting conductors and custom building the matrix or array of gradient control wires at location. There is a need for a gradient control system whereby a prefabricated array or matrix can be easily installed between underground conductors and surface-level equipment to mitigate induced electrical potentials and currents. There is a need for a gradient control system that does not require the manufacture of the grid at location. There is also a need for a gradient control ground grid that provides both electric potential mitigation and cathodic protection to the underground conductor to which it is electrically connected.